Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within a small ceramic block. The small ceramic block is passed over the disc in a transducing relationship with the disk. The transducer can be used to read information representing data from the disc or write information representing data to the disk. When the disc is operating, the disc is usually spinning at relatively high RPM. These days common rotational speeds are 7200 RPM. Some rotational speeds are as high as 10,000 RPM. Higher rotational speeds are contemplated for the future. These high rotational speeds place the small ceramic block in high air speeds. The small ceramic block, also referred to as a slider, is usually aerodynamically designed so that it flies over the disk. The best performance of the disc drive results when the ceramic block is flown as closely to the surface of the disc as possible. Today's small ceramic block or slider is designed to fly on a very thin layer of gas or air. In operation, the distance between the small ceramic block and the disc is very small. Currently "fly" heights are about 0.5-1.0 microinches. In some disc drives, the ceramic block does not fly on a cushion of air but rather passes through a layer of lubricant on the disk.
Information representative of data is stored on the surface of the memory disk. Disc drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disc is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
Disc drives have actuator assemblies which are used to position the slider and transducer at desired positions with respect to the disk. The slider is attached to the arm of the actuator assembly. A cantilevered spring, known as a load spring, is typically attached to the actuator arm of a disc drive. The slider is attached to the other end of the load spring. A flexure is attached to the load spring and to the slider. The flexure allows the slider to pitch and roll so that the slider can accommodate various differences in tolerance and remain in close proximity to the disk. The slider has an air bearing surface ("ABS") which includes rails and a cavity between the rails. The air bearing surface is that portion of the slider that is nearest the disc as the disc drive is operating. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disk. At the same time, the air rushing past the depression in the air bearing surface produces a negative pressure area at the depression. The negative pressure or suction counteracts the pressure produced at the rails. The different forces produced counteract and ultimately fly over the surface of the disc at a particular fly height. The fly height is the thickness of the air lubrication film or the distance between the disc surface and the head. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation.
Disc surfaces have asperities or bumps which interfere with the flying characteristics of the data transducer head. The asperities can also interfere with the read and write operations of the data head. In operation, the data transducer head can come into contact with asperities while the head flies above the surface of the disc. Potentially, this undesirable contact can cause data written to a particular location on a disc to be lost. Large asperities may cause a catastrophic head crash. Other smaller asperities may cause a shift in the signal read back from a transducer head. The shift needs to be corrected or pulled down so that the signal representing information stored to the disc can be read back without error.
In an ideal world, the surfaces of the disc would be flat so that a transducing head could fly over the surface without having to encounter any asperities. In the real world asperities exist on the disc surfaces so the disc surfaces are typically tested initially to determine if the disc should be rejected. In other words, if the disc is not smooth enough so that a fly height can be maintained, the disc is rejected. If the disc is substantially smooth or flat, the disc will still generally have some asperities. The asperities on a disc are located using a defect scanning process.
Disc asperities which are located in the factory during a defect scanning process can be recorded in a disc drive's primary defect list so that the drive does not store data at those locations. Known asperity detection techniques use sensors (such as MR sensors or piezoelectric sensors). Such known asperity detection techniques rely both on the flying characteristics of the heads and upon the thermal response from friction induced head/asperity contact. If one sensor is used without the other, there is a possibility that all types of asperities may not be found. The energy of the impact or amplitude detected by an MR or other sensor is calibrated to determine the asperity characteristics such as height of the asperity. By calibrating the slope and duration of the resistance change waveform to a range of asperity heights and characteristics, the height of a particular asperity can be determined by detecting the momentary change in resistance of the sensor after contact.
However, the voltage signals corresponding to the impact of a sensor element with an asperity include components of noise, air bearing excitation, and other vibrations or excitations which may detract from the accuracy of calibrating the height of an asperity based upon the voltage signal from an MR sensor element or a piezoelectric sensor element after contact with the asperity.
Additionally, such devices require that the disc surface be scanned at various fly heights of the head so that various sizes of asperities can be detected to map the entire range of defects. As the speed of rotation of the disc is changed, the response of the specially-designed heads also changes. For example, if the speed is reduced, the energy of impact is reduced, thus making it more difficult to calibrate the defect size and height.
In order to certify that a magnetic disc is adequately smooth for use in a disc drive system, testing is performed on the disc. One type of testing is performed by utilizing a test slider having a piezoelectric element bonded thereon. When any part of the slider contacts a protrusion on the surface of the disc, the slider will vibrate from the impact. Generally, the piezoelectric element is useful in finding larger asperities such as a blister on a disc. The piezoelectric element bonded on the slider senses these vibration forces acting on the slider, exhibiting a voltage between its terminals representative of the forces experienced by the element. If the vibration force sensed by the piezoelectric element exceeds a predetermined design level, or if vibration occurrences exceed a predetermined design frequency, then the disc media under test is not adequately smooth to be used in applications.
There are several problems involved with utilizing piezoelectric test sliders to test the smoothness of a disc. By bonding the piezoelectric element to the slider, the piezoelectric element loses some of its sensitivity to forces acting on the slider, since some of these forces may be absorbed through the bond to the piezoelectric element. Piezoelectric test sliders are therefore relatively insensitive to narrow defects and asperities in the disc being tested. In addition, the process of bonding a piezoelectric element to the slider affects the aerodynamic characteristics of the slider, which are desirably tightly controlled. Finally, the process of assembling a piezoelectric element on a slider is a tedious and expensive undertaking, and is not standard since piezoelectric elements are not employed on actual read/write heads. Changes in the process of manufacturing the slider must be made, which makes manufacturing test sliders less efficient.
Another type of testing is performed by equipping a test slider with thermal asperity sensor, such as a MR transducer. A thermal change occurs in the sensor upon detecting a defect or asperity in the disc at the transducer, which changes the resistance of the MR element and thereby indicates the presence of a defect on the disc. The defect or asperity may be a depression or a rise (bump) in the disc surface, as each affects the resistance characteristics of the MR element oppositely. However, if the transducer (which is positioned on a rail at the trailing edge of the slider) does not directly confront the defect on the disc, the MR sensor is unable to detect the presence of the defect. Thus, for wide defects, the slider may "bounce" over the defect after contacting it near the leading edge of the slider, the force of which would affect the height of the thermal asperity sensor over the surface of the disc and thereby cause errors in detecting the defect.
One solution to the problems presented in testing the smoothness of a disc has been to perform two separate glide tests, one with a piezoelectric test slider and one with a separate thermal asperity test slider. The piezoelectric slider is used for best performance in detecting "short and fat" defects on the disc, while the thermal asperity slider is used for best performance in detecting "tall and thin" defects in the disc.
However, additional problems are presented by using separate piezoelectric test sliders and thermal asperity test sliders. Greater time and effort is involved in performing two separate tests. Also, the potential for inaccuracies in measurements is present, since the piezoelectric test slider may not have identical flying characteristics as the MR test slider.
Current solutions may include placing both a piezoelectric and MR sensor on the slider so that two tests may be performed at once. However, the manufacturing time for producing such a glide slider is long. In addition, the process for making such sliders is generally complex. The resulting slider is less than reliable since there are more potential areas for failure. Still another problem is that an acoustic emission sensor is typically placed along the rails. As is well known, impact of media asperities with slider rails during disc operation also results in mechanical vibrations in the slider body, which may be detected by an acoustic emission sensor (AES). The vibrations that are easiest to detect are the fundamental resonance modes, due to the magnitude of their vibration. In a paper by K. O'Brien and D. Harris, Head/Disk Interface Contact Detection using a Refined Acoustic Resonance Technique, presented at Joint SAME/STLE Tribology Conf., Oc. 8-11, 1995, the first three resonance modes in a 50 s slider were determined to be 687.7, 902.4, and 1381 kHz. In the first two modes, the largest deformation occurs along the slider edge in the direction of the rails, while in the third mode the largest deformation is along the trailing and leading edges.
As a result, the third mode vibrations are not easily detected with acoustic emission sensors positioned in or near the rails of the slider.
There is a need in the art for an integrated thermal asperity and piezoelectric sensing device to allow testing of disc smoothness with a single test slider that is simply designed and provides reliable measurements. There is also a need for integrated thermal asperity and piezoelectric sensor device that can be easily manufactured. There is also a need for an integrated thermal asperity and piezoelectric sensing device that will more accurately sense the various asperities on a disc. There is still a further need for an integrated thermal asperity and piezoelectric sensing device that is more sensitive to the third mode of resonance of a slider.